Magneto‐Orientation of Magnetic Double Stacks for Patterned Anisotropic Hydrogels with Multiple Responses and Modulable Motions

Abstract Reported here is a multi‐response anisotropic poly(N‐isopropylacrylamide) hydrogel developed by using a rotating magnetic field to align magnetic double stacks (MDSs) that are fixed by polymerization. The magneto‐orientation of MDSs originates from the unique structure with γ‐Fe2O3 nanoparticles sandwiched by two silicate nanosheets. The resultant gels not only exhibit anisotropic optical and mechanical properties but also show anisotropic responses to temperature and light. Gels with complex ordered structures of MDSs are further devised by multi‐step magnetic orientation and photolithographic polymerization. These gels show varied birefringence patterns with potentials as information materials, and can deform into specific configurations upon stimulations. Multi‐gait motions are further realized in the patterned gel through dynamic deformation under spatiotemporal light and friction regulation by imposed magnetic force. The magneto‐orientation assisted fabrication of hydrogels with anisotropic structures and additional functions should bring opportunities for gel materials in biomedical devices, soft actuators/robots, etc.


Experimental Section
Materials N-isopropylacrylamide (NIPAm) was used as received from Tokyo Chemical Industry Co., Ltd.
N,N′-methylenebis(acrylamide) (MBAA) was purchased from Aladdin Chemistry Co., Ltd. Lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP) was synthesized according to the reported process. [S1] MDS was synthesized by adapting the reported procedure. [S2] The content of -Fe2O3 was fixed at 16.6 wt%. Aqueous suspensions of MDS (1 wt%) were prepared by adding prescribed amount of MDS power to water. The mixture was oscillated with a speed of 120 r.p.m. for 24 h at room temperature, which led to repulsive osmotic delamination into a ferronematic suspension consisting of singular MDSs with a sandwich-like structure. Millipore deionized water was used in all the experiments.

Synthesis of magnetic anisotropic hydrogels
The precursor suspension was prepared by dissolving a prescribed amount of NIPAm (1 mol/L), MBAA (3 mol%, relative to NIPAm), and LAP (6 mol%, relative to NIPAm) in the homogeneous suspension of MDS (1 wt%). After injecting the aqueous precursor suspension into the reaction cell consisting of a pair of parallel glass substrates with silicon spacer of a specific thickness, a static or rotating magnetic field was applied to direct the alignment of MDSs. After the magnetic orientation for a short while, the reaction cell was immediately exposed to UV light for 10 s to initiate the polymerization and crosslinking to obtain the anisotropic hydrogel.
The isotropic hydrogel containing randomly dispersed MDSs was prepared according to a similar protocol. After injection of the precursor, the reaction cell was oscillated with a speed of 60 rpm for 2 h at an elevated temperature to accelerate the structural relaxation of the shear-induced alignment of MDSs. Then, the reaction cell was cooled down to room temperature and placed under UV light irradiation for 10 s to trigger the polymerization. The obtained hydrogel was incubated in water to achieve the equilibrium state.
Patterned anisotropic hydrogels with complex ordered structures were fabricated by a multi-step magnetic orientation of MDSs and photolithographic polymerization. After the magnetic orientation of MDSs in the precursor suspension, the reaction cell was exposed to UV light for 10 s through a photo mask. After rotating the reaction cell to a certain angle, the rotating magnetic field was applied for 20 s, and the sample covered with another photo mask (or without photo mask) was exposed to UV light for 10 s, resulting in polymerization of the precursor at specific regions. The process was repeated until the whole precursor suspension was polymerized to fix the ordered structures of the MDSs. The obtained patterned hydrogel was incubated into a large amount of water for several days to remove the residuals and achieve the equilibrium state.

Locomotion of the stripe-patterned hydrogels
Motions of the stripe-patterned hydrogel were realized under cyclic scanning of a laser beam (wavelength: 520 nm; intensity: 2.34 W/cm 2 ; spot diameter: 7 mm) from left to right of the rectangular hydrogel film (dimensions: 15 mm × 5 mm × 0.6 mm) placed on the polyvinyl chloride (PVC) substrate. The magnetic force was applied and adjusted by placing a N52 magnet (dimensions: 10 cm × 1 cm × 0.5 cm) under the PVC substrate with a specific distance. The locomotion was recorded by a digital camera with a cut-off filter (550-1100 nm) to filter out the strong green light.

Characterizations
The surface topography of MDSs was determined by atomic force microscopic ( Absorption spectra were obtained by a UV-1800 spectrometer (Shimadzu Corp., Japan) at room temperature. The anisotropic gel was kept in a quartz cuvette with an optical path of 1 mm for measurement. To monitor the photothermal effect of the hydrogel, the localized temperature under irradiation of green light (520 nm) was measured by an infrared imager (Fotric 285).
The birefringent photos of MDS suspensions (1 wt %) were taken under a pair of polarizing films. The anisotropic hydrogels were observed under a POM (LV100N POL, Nikon) with and without a 530 nm tint plate. The gels with thickness of 0.5 mm were cut into strips with a width of ~0.5 mm for the cross-section observations. SAXS measurements were conducted on Xeuss SAXS system (Xenocs SA) with X-ray wavelength of 0.154 nm and beam spot of 172 × 172 µm 2 . The distance between the sample and the detector was 1371 mm. The orientation degree (π) of MDSs in the hydrogel was calculated according to the equation of π = (180-H)/180, where H is the half width of the peak of the azimuthal plot from the selected equatorial reflection.
The variations in // and ⊥ direction of the hydrogel prepared with rotating magnetic field are calculated as S (//) = L1/L0, and S (⊥) = W1/W0, respectively, in which L and W are the dimension in // and ⊥ directions. The subscript numbers 1 and 0 correspond to the deformed state and the original equilibrated state, respectively. The dimensions of hydrogels were analyzed from the snapshots of a movie that recorded the fast shape deformation of the gel after being transferred from a 25 °C water bath into a 40 °C water bath or being directly irradiated under 520 nm green light (intensity, 2.34 W/cm 2 ) at room temperature.
The mechanical properties of anisotropic and isotropic hydrogels containing MDSs were measured at room temperature using a tensile tester (Instron 3343). The anisotropic gel was cut into dumbbell-shaped samples, with a gauge length of 12 mm and a width of 2 mm, along the // or ⊥ direction. The isotropic hydrogel (without long-range orientation of MDSs) was also tested for comparison. Tensile tests were performed at a stretching rate of 100 mm/min. Young's modulus (E) (calculated with a strain below 7%), tensile breaking stress (σb), and breaking strain (εb) were obtained from three parallel measurements.         Figure S11. Schematic of the tensile directions of the equilibrated hydrogel prepared with rotating magnetic fields. Figure S12. POM images of the MDS-containing isotropic hydrogel sheet observed from the top (a) and two orthogonal cross-sections (b, c). Thickness of gel: 1 mm. For the synthesis of isotropic gel, the reaction cell containing the precursor is oscillated with a speed of 60 rpm for 2 h at an elevated temperature to accelerate the structural relaxation of the shear-induced alignment of MDSs, which is followed by cooling to room temperature and then polymerizing under UV light irradiation. Figure S13. Absorption spectra of the hydrogel with 1 wt% of MDS prepared with rotating magnetic fields. Figure S14. The maximum contraction difference in parallel (//) and perpendicular (⊥) direction incubated in warm water (40 o C) or irradiated by a green laser. Figure S15. POM images (a), scattering intensity-azimuth plots (b), and orientation degree of MDSs in the anisotropic hydrogel after incubation in water for 7 days and 6 months, as well as after 10 cycles of heating-cooling treatments. Thickness of gel: 1 mm.      The orientation degree of nanosheets was relatively low; locomotion had not been investigated. S8